In order to finish a paper or cardboard web it is known to coat it with a water-base layer of pigment. The pigment layer contains as pigment chalk, calcium carbonate, and the like with additives such as CNC, starch, and the like.
Parent German application 197 16 647 describes a method and apparatus for applying a pigment coating to a paper or cardboard web where the coating(s) having a particle content by weight of 30% to 72% is applied to the web as a free-falling curtain by a slot nozzle arranged transversely above the web-travel direction. The application from a free-falling curtain has the advantage that no extra is applied so as not to overload a following dosing element to produce a light application. In order to prevent a thinning of the edges of the free-falling curtain according to one embodiment both ends of the slot of the slot nozzle are provided with guide elements extending down to near the web and serving to prevent thinning of the edges of the free-falling curtain. Sheet-metal guide elements are preferred.
A typical method of applying a pigment coating to a paper web with a free-falling curtain is known from European 0,517,223. In this publication there is no discussion of how to deal with the problem of the curtain thinning.
In order to stabilize a free-falling liquid curtain while coating a passing web, European 0,115,621 describes arranging to both sides of the curtain so-called curtain maintainers that are provided along their entire vertical length with a manifold passage and output for directing a secondary liquid toward the curtain. The secondary liquid forms a triangular and flat stabilizing liquid bridge between the curtain and the curtain maintainers. The described method deals with coating of PE-coated paper with glycerine or aqueous gelatins for producing films, not for applying a pigment-containing coating dye.
It is an object of the invention to improve on stability and uniformity of the curtain in the known method of applying a pigment coating to a paper or cardboard web.
A further object is to provide an apparatus for carrying out the method according to the invention.
These objects are attained according to the invention in a method wherein a web is passed in a horizontal web travel direction beneath a slot nozzle extending transversely of the direction. A pigment-carrying liquid is ejected at a flow speed from a slot of the nozzle to form a downwardly free-falling curtain extending across the web. A vertical inner face of a guide element is oriented at each outer edge of the curtain with each face extending from a respective end of the slot downward to adjacent the passing web. An upper region of each of the inner faces is fed to a respective partial stream of the liquid such that the partial streams flow down the respective inner faces at a speed at least equal to the curtain flow speed and merge with the outer edges of the curtain.